Fibrous metal filaments

ABSTRACT

A METALLIC FILAMENT WHICH HAS AN EFFECTIVE DIAMETER OF LESS THAN 50 MICRONS AND IS FORMED WHILE SURROUNDED BY A SUBSEQUENTLY REMOVED SACRIFICIAL MATRIX. THE FILAMENT HAS A PRESELECTED PERIPHERAL SURFACE VARYING FROM SUBSTANTIALLY SMOOTH TO RE-ENTRANT AND A PRESELECTED SURFACE TO VOLUME RATIO. THE AREA OF THE FILAMENT ALSO HAS A CONTROLLED NON-UNIFORMITY ALONG THE LENGTH THEREOF WHICH PROVIDES AN ACCEPTABLE DIMENSIONAL TOLERANCE. THE METALLIC FILAMENT MAY BE SUBSTANTIALLY ONE METAL, BIMETALLIC OR TUBULAR.

J. A. ROBERTS ET AL FIBROUS METAL FILAMENTS 9 Sheets-Sheet l Filed Jan.29, 1970 O. 17, 1972 1 A ROBERTS E'I'AL 3,598,3

FIBROUS METAL FILAMENTS Filed Jan. 29, 1970 9 Sheets-Sheet 2 J. A.ROBERTS EVAL 3,698,863

FIBROUS METAL FILAMENTS Oct. 17, 1972 A9 Sheets-Sheet 4 Filed Jan. 29,1970 o? oo.

N202 XFCRE L A S T R E B O R A. J.

FIBRoUs METAL FILAMENTS 9 Sheets-Sheet Filed Jan. 29, 1970 OCt. 17, 1972L A, ROBERTS ETAL 3,698,863

FIBROUS METAL FILAMENTS 9 Sheets-Sheet 6 Filed Jan. 29, 1970 ,OGL 17,1972 1 A RQBERTS ETAL 3,698,863

FIBROUS METAL FILAMENTS 9 Sheets-Sheet 7 Filed Jan. 29, 1970 Oct. 17,1972 J. A. ROBERTS ETAL FIBRoUs METAL FILAMENTS 9 Sheets-Sheet 8 FiledJan. 29. 1970 United States Patent O U.S. Cl. 29-183.5 28 ClaimsABSTRACT F THE DISCLOSURE A metallic filament which has an effectivediameter of less than 50 microns and is formed while surrounded by asubsequently removed sacrificial matrix. The filament has a preselectedperipheral surface varying from substantially smooth to re-entrant and apreselected surface to volume ratio. The area of the filament also has acontrolled non-uniformity along the length thereof which provides anacceptable dimensional tolerance. The metallic filament may besubstantially one metal, bimetallic or tubular.

This application is a continuation-in-part of our copending applicationSer. No. 742,010, filed July 2, 1968, now Pat. No. 3,505,039, a divisionof application Ser. No. 348,326, filed Mar. 2, 1964, and issued July 23,1968, as U.S. Pat. No. 3,394,213.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelated to fine metal filaments and in particular to controlling thecharacteristics of solid and bimetallic filaments.

Prior art In our original application, of which this is acontinuation-in-part, methods for simultaneously forming a plurality ofmetal filaments while surrounded by a matrix material were disclosed.Products made by our processing were also disclosed. In addition, it wasdisclosed that economic manufacture of filaments could be achieved byusing high ratios of filament to matrix materials. Webber and Wilson intheir U.S. Pats. Nos. 3,277,564 and 3,379,000 and owned by the assigneehereof, disclosed another fine metal filament and methods for makingtows of fine metal filaments. However, it would be highly desirable toprovide fine metal filaments with preselected characteristics not taughtor disclosed by prior teachings.

SUMMARY OF THE INVENTION Introduction-As an introduction (and for easeof understanding) it is necessary to define the units of measurementused in this disclosure. In standard metal working, i.e., machining,drawing and the like, parts are made to a desired dimension. Because nopiece of equipment is perfect, a variation in the exact dimension isalways permitted. This variation is commonly referred to as totaltolerance or allowance. If, for example, the final diameter of a colddrawn wire is 0.005 inch, then this dimension might have a tolerance ofi0.0005 inch. Thus, the total tolerance or allowance would be 0.001inch. However, since the filaments that are the subject of thisinvention are formed while surrounded by a sacrificial matrix material,the co-working of the filaments and the matrix create a filament whereinsuch a type of measurement is not applicable in many instances. Thefilaice ments that are provided can have a preselectable irregulargeometric cross-sectional shape (regular geometric shape being definedas a square, circle, hexagon, etc.). Therefore, the size control measureused in this disclosure is the statistical term coefficient ofvariation, which is described below.

It is fully contemplated to be within the scope of this invention that afilament can have a controlled cross-sectional area that is non-uniformalong the length thereof but is uniform within a specied range. Thisrange is defined by the coefficient of variation; and the coefficientvariation for the filaments which was the subject to this disclosure isapproximately less than 25 percent. 'I'he coefficient of variation isWritten as a/E and 5:2x/n

where x is the area of each cross section taken along the length of afilament and n is the number of areas taken. In order to find thecoefficient of variation, the standard deviation (having the same unitsas x) is calculated by using the equation:

Therefore, the term coefficient of variation as used in this disclosuremeans the statistical coefficient of variation of cross sectional areastaken along the length of a filament substantially perpendicular to theaxis of the filament.

The filaments which are the subject of this disclosure can be providedwith an increased surface area and in describing the increase in surfacearea obtained, it is necessary to relate the increase to a standard. Thestandard is taken to be the surface area of a unit length cylinderhaving a cross section area equal to the cross section area of thefilament. The enhancement in surface to volume ratio is defined as thedifference between the surface area of a filament of unit length and thesurface area of the equivalent cylinder of unit length divided by thesurface area of the equivalent cylinder of unit length. This enhancementis expressed as a percentage.

Summary.-The advances in metal filament (or fiber) technology disclosedby Webber et al. and by Roberts, et al. were the basis for the formationof a commercial metal filament industry.

In this Idisclosure new and novel filament forms will be disclosed whichfunction by providing preselected properties hitherto unobtainable andyet highly desirable.

This invention relates to fine metal filaments and is concerned with newand novel filaments that have preselected peripheral surfaces,controlled uniformity, preselected `urface to volume ratios andfilaments that are bimetal- The present invention is concerned withproviding highly desirable metal filaments by selecting the properratios of preselected matrix and filament materials to achieve:economical manufacture of such filaments; preselected surface volumeratios of such filaments; preselected surface periphery of suchfilaments; and, maintaining at low levels the coefficient of variationof the cross-sectional area.

Thus, a principal object and feature of the present invention is toprovide a new and improved metal filament formed while in a sacrificialmatrix material and having a preselected surface to volume ratioachieved by the proper selection of filament and matrix materials.

Another object of the invention is to provide a filament having smoothor rough peripheral surface determined by the metallurgicalcharacteristics and physical propertes of the filament and matrixmaterials and their interaction during co-working.

Another object of the invention is to provide a metallic :filamentwherein a series of cross-sectional areas taken along the length of saidfilament substantially perpendicular to the axis of said filamentexhibits a coefficient of variation from 1% to less than 25 percent.

Still another object o-f the invention is to provide for utilizing apreselected ratio of matrix material to filament material to achieve aneconomically manufactured metal filament when the filament has anunburnished, unmachined outer surface, is fracture free and has acyclical geometric variation along the length thereof, yet remainingunder a 25 percent coefficient of variation.

And still another object of the invention is to provide a filamenthaving a non-uniform cross-sectional area, but a controlled uniformityWithin a coefficient of variation from 1 percent to less than 25 percentwherein the coefficient of variation is a function of the combinationof: (a) the volume ratio of the matrix to filament material; (b) theamount and temperature of the hot working and/ or cold working betweenanneals (if desired) imparted to the starting filament and matrixmaterials; and, (c) the choice of matrix material for a preselectedfilament material.

Still a further object of the invention is to provide a bimetallicfilament.

The above and other further objectives and features will be more readilyunderstood by reference to the following detailed descriptions and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transverse cross-sectionof a metal wire from which a filament may be formed in following themethod embodying the invention;

FIG. 2 is a transverse cross-section of the wire disposed within acoaxial sheath as in a first step of the method;

FIG. 3 is a perspective view of a plurality of sheathed wires disposedin a cylindrical housing in a subsequent step, the housing being brokenaway to illustrate the bottom portion thereof;

FIG. 4 is a reduced vertical section of a compact means illustrating acompaction of the assembly of sheathed wires in the cylindrical housingto define a compacted billet;

FIG. 5 is a reduced vertical section of another form of compacting meansillustrating another method of reducing the diameter of the assembly todefine a compacted billet;

FIG. 6 is a top plan view of a plurality of sheathed Wires in a modifiedhousing having a hexagonal internal cross-section as by the provisiontherein of sector shaped spacers;

FIG. 7 is a top plan view of a modified arrangement of the sheathedelements in a cylindrical housing with spacers disposed between thesheathed elements to effectively minimize the voids therein;

FIG. 8 is an exploded vertical section illustrating the arrangement ofthe housing subsequent to the installation of the sheathed wires thereinand prior to the securing of the end plug across the open end thereof;

FIG. 9 is a vertical section illustrating the arrangement of thesheathed wires in the housing with the end plug secured across the openend of the housing;

FIG. l0 is a vertical section illustrating the step of evacuating andsealing of the housed sheathed wires to define the primary billet;

FIG. 11 is a fragmentary diagrammatic elevation of the billet during asubsequent hot extrusion step;

FIG. 12 is a diagrammatic elevation of the extruded billet with suitablecutting means acting to remove the opposite ends of the extruded bundle;

FIG. 13 is a diagrammatic elevation illustrating the cutting of theextruded billet into a plurality of shorter lengths;

FIG. 14 is an elevation of one of the short lengths being provided witha replacement plug at each of its opposite ends;

FIG. 15 is a fragmentary side elevation of a short length being furtherhot extruded to reduce the diameter thereof;

FIG. 16 is a side elevation of the original billet being reduced indiameter as by hot rolling means in lieu of or subsequent to the hotextrusion means of FIG. 1l;

FIG. 17 is a fragmentary diagrammatic vertical section illustrating thecold drawing of the hot formed billet in a subsequent step;

FIG. 18 is a vertical cross section of a tank wherein the drawn billetof FIG. 17 is disposed to be acted upon by a suitable fluid within thetank to remove the sheathing and housing material from the billet;

FIG. 19 is a resultant tow of filaments embodying the invention;

FIG. 20 is a transverse cross section of a metallic tubular element fromwhich a tubular filament may be formed in accordance with the invention,a filler being disposed within the tubular element as in a first step ofthe method;

FIG. 21 is a transverse cross-section of the filled tubular elementdisposed within a tubular sheath as in a second step of the method;

FIG. 22 is a transverse cross section thereof constricted to provide atight assembly of the filler, tubular element, and sheath as in a thirdstep of the method;

FIG. 23 is a perspective top view of the filled and sheathed tubularelements disposed within a cylindrical housing for subsequent sealing,hot forming, and drawing steps as illustrated in FIGS. 7 through 17;

FIG. 24 is a fragmentary enlarged perspective view of a tubular filamentformed by the method;

FIG. 25 is a fragmentary enlarged perspective view of a bimetallicfilament;

FIG. 26 is a transverse cross section of a can with sheathed elementspacked in a hexagonal array having spacing shims between the faces ofthe hex and the inside diameter of the can;

FIG. 27 is a fragmentary enlarged longitudinal cross section of twonon-uniform filaments in a matrix material that exceed a 25% coefiicientof variation;

FIG. 28 is a fragmentary enlarged longitudinal cross section ofdiscontinuous filaments in a matrix material;

FIG. 29 is a graph showing the regions of gross nonuniformity andbackground non-uniformity when plotted on a chart wherein thecoefficient of variation is the ordinate and the degree of deformationis the abscissa with Wc defined as the critical deformation and thestart of the gross non-uniformity region;

FIG. 30 is an array of three graphs showing gross and backgroundnon-uniformity for different matrix material having the same ratio ofmatrix to filament material.

FIG. 3l is a graph with different curves for different ratios of thesame matrix material to the same filament material wherein thecoefficient of variation is the ordinate, the degree of deformation isthe abscissa;

FIG. 32 is a graph showing regions for stainless steel filament materialand different matrix materials wherein the ordinate is the volume ratioof matrix to total composit material and the abscissa is the criticaldegree of deformation;

FIG. 33 is a photomicrograph at 605x magnification of a compositeshowing filaments of substantially Ti- 55A (alpha-titanium) materialstill surrounded by Monel metal matrix material; the composite is asextruded;

FIG. 34 is a photomicrograph at 245 magnification of the composite ofFIG. 33 wherein the composite has been cold worked;

FIG. 35 is a photomicrograph at 125 magnification of a composite showingfilaments of substantially Ti-6Al- 4V (alpha and beta titanium) materialstill surrounded by an AISI 1010 steel matrix material; the composite isas extruded;

FIG. 36 is a photomicrograph at 240x magnification of a compositeshowing filaments of substantially Ti-6Al- 4V (alpha and beta titanium)material still surrounded by an AISI 1010 steel matrix material; thecomposite is as extruded;

FIG. 37 is a photomicrograph at 240 magnification of the composite ofFIGS. 35 and 36 wherein the composite has been cold worked;

FIG. 38 is a photomicrograph at 240 magnification of a composite showingfilaments of substantially Til3V-l1Cr-3Al (beta titanium) material stillsurrounded by an AISI 1010 steel matrix material; the composite is asextruded;

FIG. 39 is a photomicrograph at 240x magnification of the composite ofFIG. 38 wherein the composite has been cold worked;

FIG. 40 is a photomicrograph at 590x magnification of the composite ofFIG. 38 wherein the composite has been cold worked;

FIG. 41 is a photomicrograph at 245x magnification of a compositeshowing filaments of substantially Nickel 270 material still surroundedby an AISI 1010 steel matrix material; the composite is in a cold workedcondition;

FIG. 42 is a photomicrograph at 605x magnification of a compositeshowing filaments of substantially Nickel 270 material still surroundedby an AISI 1010 steel matrix material; the composite is in a cold workedcondition;

FIG. 43 is a photomicrograph at 240x magnification of a compositeshowing filaments of substantially Nickel 270 material still surroundedby an Aluminum 1100 matrix material; the composite is in a cold workedcondition;

FIG. 44 is a photomicrograph at 590x magnification of a compositeshowing filaments of substantially Nickel 270 material still surroundedby an Aluminum 1100 matrix material; the composite is in a cold workedcondition;

FIG. 45 is a photomicrograph at 240x magnification of a compositeshowing laments of substantially Nickel 270 material still surrounded byan Aluminum 5052 matrix material; the composite is in a cold workedcondition;

FIG. 46 is a photomicrograph at 600)( magnification of a compositeshowing the same filaments of FIG. 45 wherein the composite is in a coldworked condition; and

FIG. 47 is a dual graph wherein part A is a plot of the averageeffective diameter of filaments in a bundle shown as the ordinate andthe degree (percentage) of deformation is the abscissa while part B isthe coefficient of variation for the same filaments also plotted vs. thedegree of deformation.

'PREFERRED EMBODIMENTS OF THE INVENTION In an exemplary embodiment ofthe invention, a tow generally designated of filaments 11, as shown inFIG. 19, is formed by a process wherein a plurality of elongatedelements, or wires, 12 are bundled in side-byside relationship and, whenso bundled, reduced in diameter by a transverse, or radial, constrictionof the wires in the bundle to provide a resultant filament of extremelysmall diameter and great length. In addition, the invention comprehendsthe forming of the filament as a tubular filament 13, as shown in FIG.24, and a bimetallic filament 113 as shown in FIG. 25, the originalelongated element in this process comprising a tubular element 14 asshown in FIG. 20.

Broadly, the invention comprehends the constriction of the bundledwires, or tubular elements, by firstly, forming the bundled wires orelements into a billet, and subjecting the billet successively to a hotforming constriction and a subsequent drawing constriction. The hotforming constriction may be alternatively by hot extrusion or hotrolling of the billet. The drawing operation may comprise a plurality ofcold drawing steps with intermediate annealing steps.

Referring now to FIGS. 1 and 2, the wire 12 is firstly enclosed in asuitable sheath 15 formed of a material having metallurgical andphysical properties differing from those of the wire 12 to permitseparation of the sheath material from the resultant filaments whendesired. However, the mechanical flow properties of the lament andmatrix material may be similar, as desired. As illustrated in FIG. 2,the original internal diameter of the sheath may be slightly larger thanthe external diameter of the wire 12 to permit facilitated insertion ofthe wire into the sheath. Alternatively, the wires 12 and the sheaths 15may =be co-reduced, such as by cold drawing through a die, to form acomposite wherein the Wire 12 is tightly encased by the sheath 15 (butnot illustrated). The thusly, loosely or tightly sheathed wires may thenbe installed in a can, or `housing 16 having a bottom closure wall, ornose plug, 17,

with the sheathed wires extending in parallel side-by-side` uprightrelationship, as shown in FIG. 3.

For improved uniform constriction of the wires in the subsequentconstricting steps, it is desirable to closely pack the sheathed wireswithin the housing 16 as by suitably compacting the assembly. Referringto FIG. 4, one method of effecting the desired compaction is by placingthe assembly in a press generally designed 18 having a liner 18adefining a cylindrical cavity closely fitting the cylindrical housing16. The lower end of the cavity is closed by a blind die 18b and theliner 18a and the blind die 18b are supported on a suitable anvil 18C. Aram 18d is provided to apply pressure on the top of the assembly wherebythe assembly is axially shortened and thereby laterally or radiallycompacted. Such compacting apparatuses are well known in the art andneed no further description herein.

Turning now to FIG. 5, an alternate method of effecting the desiredcompaction of the assembly is shown to comprise the compaction of theassembly by means of an extrusion apparatus generally designated 118. Inapparatus 118 an extrusion die 118g is provided through which theassembly is longitudinally forced by means of a suitable pressureapplying element 118b. Only a small amount of constriction of theassembly is effected by die 118a so that only an elimination of thevoids in the assembly is effected in this step.

IReferring now to FIG. `6, a method of facilitating the compaction ofthe assembly is illustrated. More specifically, the internalconfiguration of the housing 16 is made to be hexagonal in transversecross section by means of a plurality of spacers 19 comprising chordalsector pieces or fiat shimstock stacked spacers 19a shown in FIG. 26.

FIG. 7 illustrates a further method of facilitating the compaction ofthe assembly. More specifically, in FIG. 7 the sheathed wires 12 areshown installed within the housing 16 with a plurality of spacers, orsuitable particulate materials such as metal powder, 21 disposed betweenthe wires. Thus, with the arrangements illustrated in FIGS. 6 and 7,less compaction of the assembly by the steps illustrated in FIGS. 4 and5 is required to provide the desired compacted billet.

Prior to the compaction steps discussed above, the sheathed wires 12 aresealingly enclosed within the housing 16 by means of an end plug 23installed across the open end 24 of the housing 16. As illustrated inFIG. 8 the end plug comprises a generally cylindrical disk which mayhave a notched portion 23a adapted to engage the upper end of thehousing in the housing closing arrangement. The end plug 23 is furtherprovided with an evacuation pipe 26 which opens through a hole 23b inthe end plug, being secured to the end plug by suitable means such asweld 26a.

With the sheathed wires 12 or composites 112 installed in the housing16, as shown in FIG. 10, the end plug 23 is secured across the openupper end 24 of the housing by suitable means such as weld 23C. Theevacuation pipe or tube 26 may be utilized during the welding of the endplug to the housing end 24 to fiush the interior of the housing duringthe welding of the plug. Upon completion of the installation of the endplug on the housing, a vacuum is applied to the pipe 26 by suitablemeans (not shown) to withdraw substantially all gas from the interior ofthe housing.

As shown in FIG. 10, when the desired vacuum condition is obtainedwithin the housing 16, the pipe 26 is pinched and welded closed as at26b to complete the sealing of the wires 12 within the housing 16. Toprovide an improved vacuum within the housing 16, the housing may bedisposed within a suitable conventional heater 27.

The resultant housed bundle comprising a packed billet 31 is nextsubjected to a hot forming process to reduce the diameter thereof in oneor more passes. As illustrated in FIG. 1l, the billet 31 may be reducedin diameter by a hot extrusion step wherein the `billet is forcedthrough heated extrusion dies 32 by a suitable pressure device 33. It isdesirable that the billet 31 be preheated to a preselected suitabletemperature and suitably lubricated for facilitated extrusion in thisstep. The rate and pressure of the extrusion is preselected to provideoptimum diametric reduction of the billet in conformity with the natureof the materials involved. In the event that a second extrusion step isemployed, the opposite ends 34 of the reduced diameter billet 35 aretrimmed (see FIG. 12) as by suitable cutters 36. Any non-uniformextruded end portions of the billet as determined by observation thereofmay be included in the ends 34 so cut from the billet. The trimmedbillet 35 is then divided into a plurality of short lengths 37, as shownin FIG. 13, by suitable means such as cutting wheels 38. Each of theshort length billets 37 is then provided with a nose plug 39 and a tailplug 40 as by welding as illustrated in FIG. 14. The short length billet37 is then reheated and passed through heated extrusion dies 41 (seeFIG. for further diametric constriction thereof to a final formed billet42.

As indicated briefly above, the hot forming of the billet 31 may beeffected by hot rolling the billet in lieu of the extrusion thereof.Thus, as shown in FIG. 16, the billet 31 may be suitably heated andpassed between suitable rolls 43. The billet may be firstly hot formedby the extrusion step illustrated in FIG. l1 and subsequently hotforming effected by hot rolling as desired. The rolls 43 are preferablyarranged to produce a hot forming constriction of the billet wherein theelements therein can be maintained in a substantially smooth circularcrosssectional configuration.

Alternatively, the elements or filaments can be formed to have a roughre-entrant periphery, if desired, such as shown in the photomicrograph,FIG. 38.

Subsequent to the hot forming steps discussed above, as shown in FIG.17, the resultant final formed billet 42 is drawn through a suitableconventional cold drawing die 44 by a suitable conventional drawingdevice 45. The billet may be successively drawn down to smaller andsmaller diameters by means of successively smaller dies 44 to producethe final composite diameter containing the desired small diameterfilaments. Annealing may be effected between the successive drawingsteps in conformity with the requirements of the metal of which thefilaments are formed. The cold drawing of the billet may be conductedsuitably to develop texture in the filaments and to work-harden thefilaments for providing improved mechanical properties thereto.

Where the hot rolling steps are employed, the final cold drawing of thebillet may be dispensed with, such as where the physical characteristicsprovided by the cold drawing are not required. Thus, the hot rollingsteps may be continued with successively smaller rolls 43 providing thedesired ultimate constrictive deforming of the billet whereby thefilaments may be made to have the desired diameter of approximately 50microns or less.

The filaments are released from the final constricted billet 46 bysuitable means such as selective chemical attack of the sheating 15 andcan 16. Thus, as shown in FIG. 18, the final billet 46 may be disposedwithin a tank 47 holding a suitable acid 48 to dissolve the sheathingand can material. Obviously, other methods of removal of the can andsheathing material may be employed; illustratively, the sheathing andcan material may be removed by electrochemical dissolution, thermalremoval, selective oxidation, mechanical removal, etc. In the final tow10 of filaments 11, as illustrated in FIG. 19, the filaments have anextremely small diameter, for example, down to approximately 0.002 inchor 50 microns or less. Where the filaments are formed by utilizing thehot extrusion process With subsequent cold drawing, the tow filamentsmay have a length of up to 50,000 feet or more, and where the towfilaments are formed by the hot rolling process, the length may be up to300,000 feet or more. It is to be understood that a continuous filamentmeans a single filament having a length of at least 5 feet as comparedto a staple filament, which is dened to mean any filament having alength of from approximately 1454, inch to approximately 24 inches andhaving an aspect ratio of at least 10 to 1.

It has been found that when the filaments are formed in a metal matrixand then released therefrom that the filaments have a scale free outersurface that is longitudinally fracture free (as compared with steelwool shaving formed by a cutting or chipping action), unburnished (ascompared with single end wire drawing) and unmachined. It is believedthat during the reducing steps the filament material is subjected to atriaxial stress system of reducing forces that is substantiallydifferent from that imposed on a single solid material deformed by thesame reducing method or process (i.e., wire drawing, rod rolling,swaging, extruding and the like).

More specifically, the wire 12 of which the filaments are ultimatelyformed may comprise a metal wire formed of a suitable material such asnickel, type 304 stainless steel, titanium and its alloys and the like.The sheathing 15 may comprise of mild steel, copper, aluminum and alloysthereof, Monel metal and the like. Several examples of material of whichthe can 16 may be formed are mild steel, Monel metal, copper and thelike. Thus, in the final step the can 16 and the sheathing material 15may be removed from the filament 12 by the use of an acid fluid 48 orother means as described above.

The following specific examples of the filament forming process and thefilaments formed thereby are intended only to illustrate the inventionand not to limit it in any way.

EXAMPLE 1 Type 304 stainless steel wires having a 0.250 inch diameterand an 18 inch length were inserted into Monel 400 sheaths (tubes)having a `0.293 inch outside diameter and a 0.253 inch inside diameterand an 18 inch length. The lower end of an AISI 1010 (mild) steel canhaving a 5.950 inch outside diameter, a 5.25 inch inside diameter and a22 inch length was fitted with a partially slip-fit included angle (45on a side) frusto-conical nose plug and welded thereto; thereby defininga closed end cavity. The can cavity was packed with two hundredsixtyeight (268) sheathed wires. An end plug was fitted and welded tothe upper end of the can; the end plug having a 1A inch evacuation tubesecured thereto. The composite can billet was evacuated to 0.1 micron ofmercury (this is 10-4 torr) at 800 F. and then sealed off. The billetwas heated to a temperature of 1800 F. for six hours in a graphitecontainer. The heated billet was extruded in a press using a heatedextrusion die (900 F.) to a diameter of 2.925 inches whereby anextrusion ratio of 4.3 times was obtained with the billet being quenchedin water immediately thereafter. The press was operated at a linear ramspeed of approximately 500 inches per minute with a 1340 ton upset forceand a 1200 ton running force. The resultant first extrusion was cut intopieces 10 inches long. A new 90 included angle nose plug was welded tothe lower end of the new inch long billet and a 1/2 inch thick end plugwas welded to the upper end thereof. The new 10 inch billet was heatedto a temperature of 1800 F. for approximately three hours in a graphitecontainer. The billet was extruded in a press using a heated extrusiondie to a diameter of approximately `0.625 inch providing an extrusionratio of 22.8 times in area. The billet was extruded at a rate of 145inches per minute with an upset force of 590 tons and a running force of560 tons with the billet again being immediately quenched in water. The0.625 inch billet was then cold drawn by 20% area reduction per pass forfour (4) passes, or v60% area reduction, between annealing steps. Thebillet was annealed several times at a temperature of 1700 F. and at arate of approximately two seconds per 0.001 inch of billet diameterduring the cold drawing schedules. The final billet (composite) diameterwas 0.016 inch. The can and the sheath were removed by a nitric acidleach providing filaments of approximately 0.0007 inch in diameter.These filaments had an average ultimate tensile strength ofapproximately 250,000 p.s.i. and an average elongation of 2.1% in thecold worked condition. In the annealed condition these filaments had anapproximate ultimate tensile strength of 109,000 p.s.i. with an averageelongation of approximately 11% in the example.

EXAMPLE 2 Type 304 stainless steel wires having a 0.080 inch diameterwere inserted into Monel 400 sheaths (tubes) having a 0.097 inch outsidediameter and a 0.085 inch inside diameter. The sheathed Wires were drawnthrough a 0.091 inch diameter die to facilitate mating of the surfacesof the stainless steel and Monel and were then straightened and cut intothree inch lengths. An AISI 1010 (mild) steel can having a 1.970 inchoutside diameter, a 1.740 inch inside diameter and an overall length of6 inches was fitted with a partially slip-fit 90 included angle (45 on aside) frusto-conical nose plug which was welded to the lower end of thecan. Two hundred and forty-two (242) of the sheathed wires were placedin the can with an end cap slip fitted into the can and welded thereto,thereby forming a billet. The end cap having a 1A inch diameterevacuation secured thereto. The billet was evacuated to less than 0.1micron of mercury at 800 F. and then sealed off. The evacuated billetwas then heated to 1800 F. for two hours in a graphite container. Thehot billet was extruded (in a press using a heated extrusion die) to adiameter of approximately 0.500 inch, whereby an extrusion ratio of 16times in area was obtained. The billet was extruded at the rate ofapproximately 65 inches per minute with an upset force of 272 tons and arunning force of 260 tons.

The 0.500 inch diameter extruded billet was cut into 4 portions. Oneportion of the billet was drawn to a final composite diameter of 0.008inch with annealing treatments of 1700 F. for 2 seconds per 0.001 inchof diameter interposed at substantially 60% area reduction intervals.

The final diameter of the resultant 242 filaments was found to beapproximately 0.00034 inch with a coefficient of variation ofcross-section area of 7.75%.

The remaining three portions of the extruded billet were all drawn to afinal composite diameter of 0.016 inch in such a manner -as to impart36%, 75% and 90% reduction of area in the respective portions after thelast anneal. In all three portions the filament diameter was found to beapproximately 0.0007 inch or 17.5 microns.

The coefficient of variation of the cross-sectional area and the averageeffective diameter of the filaments of the three composites are shown inFIG. 47 wherein part A of the graph shows the effective diameter as theordinate and the degree of deformation as the abscissa and p-art B thecoefficient of variation (t1/2E) also as the ordinate. As shown on thisgraph, the coefficient of variation of the filament area is well withinthe self-imposed 25% limitation. The average effective diameter of thefilament was calculated by equating the average filament area to acircle having the same area and then solving for the diameter of thecircle. This diameter is the Kaverage effective diameter of thefilaments. It can be seen from this graph that the effective diameterfor the 36%, 75 and deformably worked filaments does not 'vary more than0.02 mil (0.0002 inch).

By using the same method the coefficient of variation of the filamentsfrom Examples 3, 4, 5 `and 6, obtained and exhibited respectivelycoefficients of variation of 8.1%, 4.46%, 11.7%, and 10%. For Example 7the 2 portions had coefficients of variation of 6.3% (Ni/Al-ll00) and13% (Ni/Al-5052).

EXAMPLE 3 Titanium-55A (commercially pure type titanium) wires having a0.080 inch diameter were inserted into Monel 400 sheaths (tubes) havinga 0.115 inch outside diameter and a 0.100 inch inside diameter. Thewires and sheaths were in an annealed condition and then drawn to acomposite diameter of 0.085 inch with the titanium core wires being0.073 inch. The drawn combination was then straightened and cut intothree inch lengths. A unitary OFHC copper can with an integral nose conewas machined wherein the outside diameter of the can was 1.630 inch, theinside diameter of the can was 0.950 inch and the overall length of thecavity of the can was 3%. inches. Ninety-one sheathed titanium wireswere packed in the can in an hexagonal array with 11 sheathed wiresconstituting the vertex to vertex spacing with six sheathed wirescomprising any and all faces. Three inches long by 0.015 inch thickcopper shims were placed adjacent to the hexagonal faces of the packedrods. Additional back-up shims were stacked between the inside diameterof the copper can and the primary shims in order to reduce the open orvoid area within the can. An end cap having a 1A inch evacuation tubesecured thereto was slip fitted into the can and welded in place so thatthere was substantially no longitudinal movement of the rods. The billetwas evacuated to 10-5 torr at 800 F. and sealed thereafter. The billetwas then heated to 1400 F. and extruded to a diameter of 0.270 inch froma 1.100 inch extrusion liner. A photomicrograph of a portion of theextruded billet is shown in FIG. 33 wherein the filaments have anapproximate effective diameter of approximately 0.020 inch. The billetwas extruded with an upset force of 56 tons. The billet was cold workedfrom 0.270 inch diameter through a series of drawing dies withintermediate anneals at 1000 F. employed after different drawingdiameters. The resultant composite was drawn to 0.0126 inch having beenreduced 50% by cold work after the last anneal. The can and surroundingMonel tubing were dissolved in nitric acid. The final diameter of theresultant filaments was found to be approximately 0.0009 inch. Aphotomicrograph of a portion of the final cold drawn composite showingfilaments with reentrant peripheries is shown in FIG. 34. Theenhancement of surface to volume ratio of these filaments is 38.8%. Thecoefficient of Variation of these filaments was found to be 8.1%.

EXAMPLE 4 Ti-6Al-4V (titanium alloy) wires having a 0.079 inch diameterwere inserted into AISI 1010 (mild) steel sheaths (tubes) having a 0.115inch outside diameter and a 0.100 inch inside diameter. The sheathedwires were then swaged to have an outside diameter of 0.0875 inch withthe titanium alloy wires having a 0.076 inch diameter; the swagedsheathed wires were straightened and were cut to 3inch lengths. An AISI1010 (mild) steel can having a 1.250 inch outside diameter, a 1.150 inchinside diameter and an overall length of 3`1/2 inches was fitted with a1A inch thick slip lit lower end cap with the cap welded thereto.Adjacent to this end cap a 90 included angle nose plug was secured tothe can by means of welding. One hundred twenty-seven sheathed rods werepacked into the can cavity in an hexagonal array. In between each offive sides of the hexagon and the circular interior of the can, twoadditional sheathed rods and two 0.080 inch diameter (mild) steel rodswere inserted. In between the sixth side of the hexagon and the interiorsurface of the can shims were packed in order to fill the remaining voidspace; therefore, the packed can contains 137 sheathed titanium alloywires. An upper end plate 1A inch thick and having a 1A inch evacuationtube secured thereto was slip fitted into the can cavity and weldedthereto; thereby forming a billet. The billet was evacuated to -5 torr.at 800 F. and then sealed off. To enhance the extruding capabilities ofthe evacuated billet, a mild steel cylinder having a 1% inch outsidediameter, a 1/2 inch inside diameter and 11A inch length was placed overthe protruding sealed evacuation tube with the cylinder then being tackWelded to the back of the billet. The billet was then heated to 1600 F.and extruded from a 1.280 inch extrusion liner through a 0.370 inch die(12 times reduction in area) when an upset force of 99 tons and arunning force of 76 tons were employed. Photomicrographs of a portion ofthe extruded billet are shown in FIGS. 35 and 36 wherein the filamentshave an effective diameter of approximately 0.024 inch. The extrudedbillet was then cold drawn through a series of cold drawing steps to afinal diameter of 0.018 inch. The billet was annealed between several ofthe drawing steps at a temperature of 1450 F. The can and the sheathing(matrix) material were then removed by chemical dissolution in nitricacid. The final diameter of the resultant 137 filaments was found to -beapproximately 0.00116 inch. A photomicrograph of a portion of the finalcold drawn composite showing filaments with re-entrant peripheries isshown in FIG. 37. The enhancement of surface to volume ratio of thesefilaments is 33.8%. The coefficient of variation was observed to be4.46%.

EXAMPLE 5 Ti-13V11Cr-3Al (titanium alloy) wires having a 0.079 inchdiameter were inserted into AISI 1010 (mild) steel sheaths (tubes)having a 0.115 inch outside diameter and a 0.100 inch inside diameter.The sheathed wires were swaged to have an outside diameter of 0.086 inchand were then straightened and cut to 31/2 inch lengths.

An AISI 1010 (mild) steel can having a 1.493 inch outside diameter, a1.370 inch inside diameter and an overall length of 4% inches was fittedwith a li inch thick lower end cap inserted into the bore of the can andwelded thereto. A frusto-conical nose plug having a 3A; inch flatportion and a 90 included angle was welded to the lower end of the canadjacent to the previously welded flush end cap. Thecan was packed with199 sheathed wires in a substantially hexagonal array with mild steelWires having diameters of 0.071 inch, 0.041 inch, and 0.028 inchinserted around the periphery of the hexagonal array to provide atightly packed can. 4In order to improve the extrusion characteristics,a 1.365 inch outside diameter by 1 inch long (mild) steel cylinder wasslip-fitted into the back of the can. A 1A; inch thick end cap was thenslipped into the upper end of the can and welded thereto, therebyforming a billet; the end cap having a '1A inch diameter evacuation tubesecured thereto. The billet was evacuated to l0F5 torr at 800 F. andthen sealed off. The evacuated billet was then heated to l650 F. The hotbillet was extruded (in a press using a heated extrusion die) from a1.530 inch diameter extrusion liner through a 0.382 inch diameter die.The billet was extruded with an upset force of 155 tons. Aphotomicrograph of a portion of the extruded billet is shown in FIG. 38wherein the filaments have an effective diameter of approximately 0.0185inch. The 0.382 inch diameter extruded billet was then cold drawn downto 0.036 inch With intermediate anneals using an annealing temperatureof l500 The can and sheathing (matrix) material were then removed bychemical dissolution in nitric acid. The final diameter of the 199filaments was found to be approximately 0.0017 inch. Photomicrographs ofa portion of the final cold drawn composite showing filaments withhighly re-entrant peripheries are shown in FIGS. 39 and 40. Theenhancement of surface to volume ratio of these filaments is 56%. Thecoefficient of variation was observed to be 11.7%.

EXAMPLE 6 Nickel 270 wires having a 0.080 inch diameter were insertedinto AISI 1010 (mild) steel sheaths (tubes) having a 0.115 inch outsidediameter and a 0.100 inch inside diameter. The sheathed wires were drawnto a 0.085 inch outside diameter with the nickel wires reduced toapproximately 0.070 inch diameter. An AISI 1010 (mild) steel can havinga 1.063 inches outside diameter, a 0.950 inch inside diameter and a 3%inch length was fitted with a -lz inch thick lower end cap inserted intothe bore of the can and welded thereto. A frusto-conical nose plug witha included angle was tack welded to the lower end of the can adjacent tothe previously welded end cap. The can was packed with 9'1 sheathedwires disposed in an hexagonal array giving 11 sheathed wires fromvertex to vertex and 6 sheathed Wires on any one side of the hexagon.Several layers of mild steel shim stock were placed between thehexagonally packed rods and the interior surface of the can. A 1A; inchthick endcap was slip fitted into the can and welded thereto therebyforming a billet; the end cap having a 2% inch diameter evacuation tubesecured thereto. The billet was evacuated to 10-5 torr at 800 F. andsealed. The evacuated billet was heated to 1600 F. and extruded to adiameter of 0.270 inch from a 1.10() inch extrusion liner at a rate ofinches per minute. The billet was cold Worked from the 0.270 inchdiameter through a series of drawing dies with intermediate anneals at1650 F. When the cold drawn composite had a 0.014 inch diameter it wascut into two portions. The first portion was subjected to a leachingoperation using phosphoric acid (H3PO4) to dissolve the can and matrixmaterial. The final diameter of the resultant nickel filaments was foundto be approximately 0.001 inch diameter. The second portion was colddrawn through another series of reducing steps to a final compositediameter of .0071 inch. The can and surrounding matrix material weredissolved in phosphoric acid. The final diameter of these secondresultant filaments was found to be approximately 0.0005 inch. Thecoefficient of variation of the 0.001 inch and 0.0005 inch filaments wasfound to be 9.45% and 7.5% respectively. FIGS. 41 and 42 arephotomicrographs of filaments in a 0.022 inch diameter composite. Thiscomposite subtends filaments with an eective diameter of 0.0016 inch andthe filaments have a coefficient of variation of 10% and an enhancementof surface to volume ratio of 31%.

For suitable softer materials, such as aluminum alloys, used as thesheathing 15 or matrix and nickel elements, it has been found that thehot extrusion can be eliminated and only cold drawing steps performed onthe billet. Thus, the following example is an illustration thereof.

EXAMPLE 7 Nickel 2704 wires having a 0.060 inch diameter were insertedinto aluminum 5052 sheaths (tubing) having a 0.115 inch outside diameterand a 0.100 inch inside diameter. The sheathed wires were drawn to a0.0475 inch diameter and were then straightened by stretching, and cutinto 31/2 foot lengths. An aluminum 5052 can having a 0.625 inch outsidediameter, a 0.555 inch inside diameter and a length of 3% feet waspacked with 97 sheathed wires. The packed can underwent cold drawing by20% reduction in area per pass. Protracted cold reduction reduced thepacked can to a 0.0071 inch outside diameter. The can and sheathing(matrix) material were then removed by chemical dissolution in causticsoda (NaOH) solution. The final diameter of the resultant 97 nickelfilaments was found to be approximately 0.0005 inch. During the colddrawing steps photomicrographs (see FIGS. 45 and 46) of the crosssection of this composite were taken when the composite diameter was0.0202 inch and the filaments were 0.0015 inch. These filamentsexhibited substantially smooth peripheries with an enhancement ofsurface to volume ratio of only 6% and a coeflicient of variation of13%. A similar composite was made using nickel 270 wires sheathed inaluminum 1100 which were encased in a Monel 400 tube. Sixty-one suchsheathed Wires were placed in the tube. The packed Monel tube was colddrawn to a diameter of 0.020 inch with the can and sheathing (matrix)material being removed by chemical dissolution. The final diameter ofthe resultant 61 filaments of this example was found to be approximately0.0019 inch and a portion thereof appeared in the photomicrographs ofFIGS. 43 and 44. These filaments exhibited extremely smooth peripherieswith an enhancement of surface to volume ratio of only 1.7%. Thecoefiicient of variation of filament cross-section area was found to be6.3%.

As indicated brieliy above, the present invention comprehends theforming of tubular and/or bimetallic filaments by a process similar tothat used in forming solid metal filaments. As illustrated in FIGS.20-24 in the forming of tubular or bimetallic filaments, the startingelongated element comprises a solid wire core 49 inserted into asuitable sheath 50 made from a material different from the Wire. Thewire 49-sheath 50 combination is then inserted into a tube 14 made froma material different from the sheath 49 material. The material used forthe wires 49, sheaths 50 and tubes 14 may have different characteristicsso that the tubes 14 can be removed from the sheath 50 and the wires(cores) 49 can also be removed from the sheath 50, as desired.

Illustratively, the wire-sheath-tube arrangement may be assembled into acomposite 51 by constrictively drawing the three components together toform intimate contact therebetween. The composites 51 are cut tosuitable length and packed in a desired array in can S2. Can 52 is thencapped at both ends, evacuated and sealed in a similar manner to themethod described above to form a packed and evacuated billet. The billetis then heated and extruded with subsequent cold drawing thereafter,thereby providing the desired tubular or bimetallic filament size. Thecold drawn billet can then be cut into either short or long lengths, asdesired. The can, tubes (matrix) and' cores may be removed from theshort length composites thereby forming a plurality of tubular elements(the original sheaths). Alternatively, either the short lengthcomposites or longer length composites may have the can and tubes(matrix) removed thereform thereby forming either short or longbimetallic filaments (original Wire and original sheath). The followingspecific examples of tubular or bimetallic filament forming processesand the tubular or bimetallic elements formed thereby are intended onlyto illustrate the invention and not limit it in any Way.

EXAMPLE 8 Copper Wires having a diameter of 0.046 inch were insertedinto Type 304 stainless steel sheaths having an outside diameter of0.088 inch and an inside diameter of 0.048 inch which were then insertedinto Monel 400 tubes having an outside diameter of 0.105 inch and aninside diameter of 0.090 inch. The wire-sheath-tube combination was colddrawn through drawing dies to give a resultant composite with an outsidediameter of 0.100 inch, the stainless steel sheath had an outsidediameter of 0.085 inch and the copper wire had an outside diameter of0.045 inch. Ther composite was straightened and cut into three inchlengths. An AIS-I 1010 (mild) steel can having an outside diameter of1.063 inch and an inside diameter of 0.920 inch and a Sil/z inch lengthwas fitted with a partially slip-fit included angle frustoconical noseplug with the nose plug welded to the lower end of the can. The can waspacked with 61 wire-sheathtube composites and an end-plug was fittedinto and welded to the upper end of the can; the end cap having a 1Ainch diameter evacuation tube secured thereto. The composite can orbillet was evacuated to l05 torr at 800 F. and then sealed off. Theevacuated billet was heated to a temperature of 1800 F. The hot billetwas extruded (in a press using a heated extrusion die) to a diameter of0.266 inch whereby an extrusion ratio of 16 times in area was obtained.The press was operated with a 70 ton upset force and a 65 ton runningforce. The extruded billet was then cold drawn to approximately a 0.172inch outside diameter. The resultant billet was then cut into suitableshort lengths of approximately 3 inches and the can, tubes, and wireswere chemically dissolved leaving tubular filaments. These filaments hadan average transverse dimension (i.e. between vertices) of approximately0.015 inch. In addition, a long portion of the cold drawn billet wastreated to dissolve the mild steel can and the yMonel tubing resultingin bimetallic filaments having the same size as the tubular filaments.These bimetallic filaments comprised a Type 304 stainless steel exteriorand a copper core with a small or trace amount of solid state diffusionoccuring at the copper-stainless steel interface. The copper corediameter was approximately 0.008 inch.

EXAMPLE 9 Monel 400 Wires having an outside diameter of 0.150 inch wereinserted into Type 304 stainless steel sheaths having an outsidediameter of 0.228 inch and an inside diameter of 0.152 inch which werein turn inserted into a Monel tube having a 0.250 inch outside diameterand a 0.230 inch inside diameter. These Wire-sheath-ttlbe compositeswere straightened and cut to 9 inches in length. An AISI 1010 (mild)steel can having a 2.950 inches outside diameter, 2.840 inches insidediameter and a 91/2 inch length was fitted With a 1A inch thick lowerend-cap inserted into the bore of the can and Welded thereto. Atruste-conical nose plug having a 2.750 inch diameter, a 90 includedangle nose with an approximately 5A; inch circular fiat portion thereonwas welded to the lofwer end of the can adjacent to the previouslywelded fiush end cap. Ninety-one composites were packed into the can inhexagonal array with mild steel metal shims placed between the hexagonalfaces of the packed rods and theinternal walls of the can in order todecrease the amount of open space therein. An upper end cap 1A; inchthick with a 1A inch diameter evacuation tube thereon was fitted intothe packed can and welded thereto. The billet was then evacuated to lessthan 10-5 torr at 800 F. and then sealed off. The billet was then heatedto 1800 F. and extruded (in a press using a heated extrusion die) to adiameter of 0.625 inch. The billet was extruded with an upset force of650 tons. The 0.625 inch billet was then cold drawn through successivedies with intermediate anneals to a composite diameter of 0.057 inch.The composite was cut in half with the first portion thereof beingsubsequently drawn to a 93.8% area reduction wherein the composite had a0.0143 inch diameter. The can and Monel tubes were then removed bychemical dissolution in nitric acid. The resultant bimetallic, stainlesssteel- Monel iilament had an overall outside diameter of approximately0.001 inch and a core diameter of approximately 0.00066 inch. During theprocessing of this bimetallic filament solid state diffusion took placebetween the Monel core and the stainless steel sheaths at the interfacethereof. The second portion of the billet was further cold reduced indiameter with intermediate anneals at 1800 F. to a composite diameter of0.028 inch and given a final anneal at 1800 F. The annealed compositewas then drawn down to 0.007 inch diameter having a cold work reductionof area of 93.8%. The can and tubing material were then removed bychemical dissolution in nitric acid. The final diameter of the resultantbimetallic filaments (stainless steel-Monel) was found to beapproximately 0.0005 inch wherein the Monel core had a diameter ofapproximately 0.00033 inch. It was found that a small or trace degree ofsolid state diffusion took place at the interface between the Monel andstainless steel. It is to be understood that the process for makingthese bimetallic filaments can be stopped after the extrusion step orany of the cold drawing steps with the bimetallic filaments releasedfrom the can and tubing (matrix) in order to obtain any desiredbimetallic filament diameter.

By this method it has been found that it is possible to form aIbimetallic filament wherein the core can vary from less than onepercent to over 80 percent of the total filament area. The bimetallicfilaments can have any desired diameter. By re-bundling theconstrictively reduced composites containing the bimetallic filaments,it is provided that such filaments may be formed concomitantly therebyproducing from as few as 2 to as many as thousands of bimetallicfilaments, as desired. The basic method also provides means for makingbimetallic filaments of any desired size having an outside diameter froml5 mils to less than one micron. Illustratively, many bimetallicfilament material combinations can be provided, such as: stainless steelover low alloy steel or mild steel, e.g. 1010, niobium over Monel,tantalum over Monel, nickel base super alloy over low alloy steel ormild steel, e.g. 1010, copper over aluminum, mild steel, e.g. 1010 overaluminum, aluminum over mild steel, e.g. 1010, gold over copper,platinum over copper, nickel over mild steel, e.g. 1010, copper overMolypermalloy, titanium over mild steel, c g. 1010, Monel overyberyllium, aluminum over magnesium, Hastalloy X over molybdenum, and thelike. It is also contemplated that the core material does not need to becompletely surrounded by the sheath (exterior) thus forming a bimetallicfilament wherein a first portion material is adjacent to a secondportion wherein the geometric cross section of the bimetallic filamentcan be substantially semi-circular shapes, rectangular shapes, circularsegmented shapes and the like. Another embodiment of this invention isthe ability to form filaments with either high or low peripheral surfacearea to volume ratios. When nickel filaments are made in an AISI 1010(mild) steel matrix as shown in FIGS. 41 and 42 wherein the effectivediameter of the filaments are approximately 0.0016 inch the crosssectional configurations of the filaments are extremely rough and thecross sectional area of the filament can be best described as having ahighly reentrant surface. It was found that the coefficient of variationof these filaments was less than 25%. The peripheral surface area to thevolume of the filament was found to be enhanced by 31.6% over thesurface area to volume of a circular sectioned element having the samecrosssection area. This highly re-entrant filament provides certaindesirable characteristics when desired, such as extreme roughness aswell as the high ratio of peripheral surface area to volume. By changingthe matrix material for the nickel filaments from mild steel to Aluminum1100 as is shown in FIG. 43 and FIG. 44, a substantially smooth nickelfilament was formed with a much lower peripheral surface area to volumeratio wherein the enhancement was only 1.7% over the surface area tovolume of a circular sectioned element having the same cross-sectionarea. In addition, it has been found that for the same filament materialit is possible to vary the surface to volume ratio by using differentmatrix materials. The nickel filaments of Examples 6 and 7 areillustrative of the ability to preselect the matrix to provide differentsurface to volume ratios for the same filament material, having suitablyadjusted the processing. Thus, it has been found that the enhancement ofthe surface to volume ratio of the filament may be preselected.

In addition the use of the same matrix material for different filamentmaterials also causes either smooth or rough peripheral .surfaces of thefilaments, `as desired. When a mild steel matrix was used to formfilaments of Ti-13Vl 1Cr-3Al (titanium alloy) a rough peripheral surfaceon the filaments was produced which may be seen in FIGS. 39 and 40. Thismaterial corresponds to the filaments described in Example 5 above. Whenan AISI 1010 (mild) steel matrix was used with Ti6Al4V to form titaniumalloy filaments as described in Example 4 above, the peripheral surfaceof the laments also exhibited reentrant features. However, theenhancement in surface to volume ratio is 70% greater for the filamentsin Example 5 compared to those in Example 4. These examples exhibit thatit is possible to preselect the filament material for a given matrixmaterial in order to obtain various peripheral surface to volume ratios.It is therefore contemplated that this disclosure provides forpreselecting the filament and matrix materials to produce a pre-selectedcross-sectional configuration of the filaments wherein either a highlyreentrant cross-sectional configuration is formed, or a substantiallysmooth and circular cross-sectional filament is formed, as desired. Ithas been found that the metallurgical characteristics of the filamentmaterial and the matrix material can be varied to preselect theperipheral surface. It has been found that when using a high ratio ofrelatively soft matrix material such as copper to a hard filamentmaterial such as type 304 stainless steel that extensive hot and/or coldreduction causes the formation of discontinuous filaments or cyclicfilaments. This discontinuity or cyclic effect is shown in FIGS. 27 and28 wherein a section of the hard filament 112 has elongated crestportions 211 and elongated valley or root portions 212 while surroundedby the soft matrix material 115 and wherein the hard filaments 112Aresembles elongated droplets surrounded by the soft matrix 115A.

It may be readily observed that if a series of cross-sections were takenalong the length of the filament 112 or the droplets 112A perpendicularto the axis 110 or 110A and the areas thereof obtained, that thecoefficient of variation of these areas would exceed 25% for bothexamples. Obviously, filaments 112 and droplets 112A would beunsatisfactory as continuous filaments or even staple filament. However,by the use of proper preselected ratios of soft or hard matrix materialto soft or hard filament material it has been found that the coefficientof variation can be controlled to a level of less than 25 therebyproviding a useable cyclic filament. In order to keep nonuniformity ofthe cross-sectional area of the filament within the self-imposedcoefficient of variation of 25 several factors are predominant incontrolling the coefficient of variation; (1) the volume ratio of matrixmaterial to the filament material, (2) the amount and temperature of thehot working and/ or the amount of cold working between intermediateanneals, (3) the choice of matrix material for a preselected filamentmaterial and (4) the distribution of the filament material in thematrix. There are basically two classes of non-uniformity in finefilaments produced by multiple end reduction; background non-uniformityand gross non-uniformity. Background non-unformity can be characterizedand defined as resulting from (l) the grain structure of the matrix andfilament materials which because anistrophy (where the physicalproperties of the metals are not the same in all crystallograpln'cdirections) of the metals will produce irregularities on the filamentsurface during the reduction processes and (2) from the presence ofnon-ductile inclusions in the filament and/or matrix materials which arenot infiuenced by the reduction processes.

Gross non-uniformity is the region where the coefficient of variationhas exceeded the background non-uniformity at higher levels ofdeformation. The magnitude of the gross non-uniformity depends upon (l)the degree of deformation performed on the specic composite betweensuccessive recrystallization heat treatments, (2) the magnitude ofdifference in mechanical flow properties of the various matrix andfilament materials in the composite,

l 17 (3) the volume ratio of the matrix and filament materials, and (4)the distribution of the filament material within the matrix. FIG. 29graphically depicts background non-uniformity and gross non-uniformitywhen plotted on a chart wherein the coefficient of variation is theordinate and the degree of deformation is the abscissa. It may be seenfrom FIG. 29` that there is a level of deformation at which the filamentcross-section area non-uniformityr begins to show a rapid increase. Thislevel of deformation is defined as the critical deformation (Wc). Oncethe filament and matrix. materials have been selected, the coefficientof variation does not change substantially with respect to the amount ofdeformation as long as there is no intrusion into the grossnon-uniformity region. However, the magnitude of the backgroundnon-uniformity is cumulative and becomes progressively greater as thefilaments become progressively smaller. The graph of FIG. 29 indicatesthat the gross non-uniformity is a function of the degree of deformationand the process steps may be chosen so as to avoid this region ifdesired. 'Ihe three graphs of FIG. 30 show the curves for type 304stainless steel filaments drawn in a copper matrix (A); type 304stainless steel filaments drawn in a mild steel matrix B); and, type 304stainless steel filaments drawn in a Monel Matrix (C). The ratio ofmatrix to filament materials for all three curves is 2:3. These curvesappear on the three graphs wherein the ordinate is the coefficient ofvariation and abscissa. is the degree of deformation. This graphindicates that for type 304 stainless steel filaments morerecrystallization heat treatments are required during the constrictingoperations when a copper matrix is used in contrast to a Monel matrix inorder to obtain equivalent uniformity. FIG. 31 is a graph wherein theordinate is the coefficient of variation and the abscissa is the degreeof deformation. Four curves are shown on this graph with curve A havinga volume ratio of matrix to filament material greater than curve B;curve B greater than curve C; and curve C greater than curve D. Thisgraph indicates that by the use of lower matrix to filament volumeratios it is possible to achieve higher degree of deformation withoutintermediate recrystallization steps in order to produce filamentshaving a cross-section wherein the coefficient of variation is within 25Therefore, it has been found that by lowering the volume ratio of matrixto filament material it is possible to achieve a more economicmanufacture of filament material because it is possible to put a muchhigher degree of deformation into the filaments between annealingoperations without going into the gross non-uniformity region. Also,since the matrix is a sacrificial material, its use in reducedquantities constitutes an economic advantage. FIG. 32 is a graph whereinthe volume ratio of matrix to total composite material is the ordinateand the critical degree of deformation (percent cold work) required tostay outside the gross nonuniformity region (We) is the abscissa. On thegraph there are three curved area regions, shown as shaded regions; (a)defining a region of copper matrix-stainless steel filament, (b)defining a region of mild steel matrix-stainless steel filaments and (c)defining a region of Monel 400 matrix-stainless steel filaments. Thedifferent regions indicate that as the volume ratio of matrix to totalcomposite material is decreased the ability to perform uninterruptedcold work to a greater degree increases. Thus it is evident that by theproper preselection of the matrix material, the filament material andthe amount of cold work to be performed on the composite, it is possibleto select the most desirable materials for the ultimate filaments to beformed. Obviously, these curves are only illustrative of the particularmaterials used, however, this same relationship will hold true for allmatrix and filament materials. Thus it can be seen that thecross-sectional area uniformity of the filaments will depend on therelationship between: (a) the volume ratio of matrix to filamentmaterial, (b) the extent of deformation between the heat treating stepsin the process, and (c) the mismatch in the mechanical flow propertiesof the filament and matrix materials. Further, the appreciation of thisinterdependency of the matrix and filament materials allows for aprocess or processes to be provided which results in a filamentcross-section area non-uniformity of less than 25% coefficient ofvariation. In addition, when a filament-matrix combination of startingmaterials is hot worked it is necessary to select a proper volume ratioof matrix to filament material so that the mismatch of mechanical flowproperties of the materials at elevated temperatures is consistent withthe desired final filament uniformity. When using composites of equalvolume ratio of Monel matrix and stainless steel filament materials andmild steel matrix and stainless steel filament materials, and when thesame hot extrusion temperature and reduction ratios are used, it hasbeen found that the filament cross-section area non-uniformity is of theorder of l5% in terms of the coefficient of variation. However, for thesame volume ratio of copper matrix to stainless steel filament materialand under similar hot working conditions the filaments produced werediscontinuous and the coefficient of variation approached whichindicates that copper, having a greater mismatch in mechanical owproperties with respect to stainless steel at the hot workingtemperature, would not perform under the conditions that Monel and mildsteel would operate. By the interplay of the parameters of the degree ofhot and/or cold wor-king, the volume ratio of matrix to filamentmaterial :and their mechanical properties, 1a definite criterion hasbeen provided for the economical and practical production of filamentsthat have a low level of cross-section area non-uniformity shown by acoefficient of variation of less than 25%.

It is to be understood that the preceding parameters are applicable tofilaments formed of one metallic material or bimetallic filaments.

Thus, the present invention comprehends an improved method of formingsolid, tubular and bimetallic filaments, preselecting the filament andmatrix materials to achieve a coefficient of variation of less than 25%,a preselected peripheral geometry, with the added feature that thefilaments may be formed in extremely long lengths.

While we have shown and described certain embodiments of our invention,it is to be understood that the invention is capable of manymodifications. Changes, therefore, in the construction and arrangementmay be made without departing from the spirit, scope and intent of theinvention as described in the appended claims.

We claim:

1. A wrought metallic filament having an effective diameter of less than50 microns formed while surrounded by a sacrificial metal matrixmaterial with the matrix substantially removed therefrom, the filamentcomprising:

(l) an area coefficient of variation of less than 25% that is defined bya series of cross-sectional areas taken along the length of thefilament;

(2) a pre-selected enhanced surface area to volume ratio, compared tothe surface area to volume ratio of a right circular cylinder ofsubstantially equal volume and equal cross-sectional area and defined bythe ratio of the peripheries of the cross-sectional areas of thefilament compared to the cylinder, the preselected surface area of thefilament defined by the metallurgical characteristics of the filamentmaterial and the matrix material; and

(3) the filament having an unmachined, unburnished,

and substantially fracture free outer surface, the surface of thefilament having a trace of the removed surrounding matrix material andhaving a texture developed by successive hot and cold working thereofwhile surrounded by the matrix.

2. The filament of 1 wherein the periphery of the filament isre-entrant.

3. The filament of claim 1 wherein the periphery of the filament issubstantially smooth.

4. The filament of claim 1 wherein the filament is bimetallic.

5. The filament of claim 1 wherein the filament is continuous.

6. The filament of claim 1 wherein the effective cross section dimensionof the filament is cyclic along the length thereof.

7. The filament of claim 1 wherein the filament is tubular.

8. The filament of claim 1 wherein the surface area to volume ratioexcess 1.00 by a range of about 1% to over 60%.

9. The filament of claim 1 wherein said metallurgical characteristics ofthe filament material are similar to those of the matrix material.

10. The filament of claim 1 wherein the metallographic characteristicsof the filament material and the matrix material are different.

11. The filament of claim 1 wherein the filament material exhibits arecrystallized structure.

12. The filament of claim 1 wherein the filament material is not arecrystallized structure.

13. The filament of claim 1 wherein the coefficient of variation iscontrolled by preselecting the ratio of the matrix material to thefilament material.

14. The lament of claim 1 wherein the coefficient of variation ismaintained substantially within the background non-uniformity region.

15. The filament of claim 1 wherein the coefficient of variation iscontrolled by the mechanical properties of the matrix and the filamentmaterials.

16. The filament of claim 1 wherein the filament material is distributedwithin the matrix material in a preselected configuration.

17. The filament of claim 1 wherein the coefficient of variation iscontrolled by preselecting the ratio of matrix and filament material,maintained substantially within the background non-uniformity region,and the mechanical properties of the matrix and the filament materials.

18. A bi-metallic filament having an effective diameter of less than 5mils comprising:

a first portion formed of a first metallic material and having a smallamount of another material present on a portion of the exterior surfacethereof; and

a second portion formed of a second metallic material adjacent to thefirst portion, the first portion and second portion having an interfacetherebetween, the filament having a texture developed by successive hotand cold working while surrounded by an after removed matrix material,the matrix material being the other material present on the firstportion.

19. The filament of claim 18 wherein said second portion ranges betweenless than 1% to approximately 80% of said total filament.

20. The filament of claim 18 wherein said second portion has a traceamount of said first portion solid state diffused therein adjacent tosaid interface.

21. The filament of claim 18 wherein said first portion has a traceamount of said second portion solid state diffused therein adjacent tosaid interface.

22. The filament of claim 18 wherein there is solid state atomicinterdiffusion between said first and second portions.

23. The filament of claim 18 wherein there is an absence of diffusionbetween the atoms of said first portion and said second portion.

24. The filament of claim 18 wherein said filament has a diameter ofunder l5 mils.

25. The filament of claim 18 wherein said second portion comprises acore substantially surrounded by said first portion.

26. The filament of claim 25 wherein said filament has an axis and across-sectional area substantially perpendicular to said axis, said areahaving a coefficient of variation of less than 25%.

27. The filament of claim 18 wherein said first portion material is analloy.

28. The filament of claim 18 wherein said second portion material is analloy.

References Cited UNITED STATES PATENTS 3,379,000 4/1968 Webber et al29-193 X 3,098,723 7/'1963 Micks 29-`183.5 2,842,440 7/1958 Nachtman etal. 29-192 RX 3,113,376 12/1963 Pfiumm et al 29-l83.5 X 3,087,233 4/1963Turnbull 29-193 UX 2,825,108 3/1958 Pond 29-193 ALLEN B. CURTIS, PrimaryExaminer

